Ideas have consequences
Social Darwinism is generally understood to use the concepts of struggle for existence and 'survival of the fittest' to justify social policies which make no distinction between those able to support themselves and those unable to support themselves.
With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health. We civilised men, on the other hand, do our utmost to check the process of elimination; we build asylums for the imbecile, the maimed, and the sick; we institute poor-laws; and our medical men exert their utmost skill to save the life of every one to the last moment. There is reason to believe that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man. It is surprising how soon a want of care, or care wrongly directed, leads to the degeneration of a domestic race; but excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed.
Capitalists all, they took what they thought were the lessons of Darwin's book and applied them to human society. Their hard-hearted conclusion, of which a 17th-century religious puritan might have been proud, was that people got what they deserved — albeit that the criterion of desert was genetic, rather than moral. The fittest not only survived, but prospered. Moreover, the social Darwinists thought that measures to help the poor were wasted, since such people were obviously unfit and thus doomed to sink.
It is hard to overestimate the influence of Darwinian and eugenic ideas in the Progressive Era. I believe one cannot fully understand the economic ideas that underwrote labor and immigration reform without also understanding the biological thought that crucially informed them.
From Darwin to Hitler
The western nations of Europe, who now so immeasurably surpass their former savage progenitors, and stand at the summit of civilisation, owe little or none of their superiority to direct inheritance from the old Greeks, though they owe much to the written works of that wonderful people. ... the anthropomorphous apes, as Professor Schaaffhausen has remarked, will no doubt be exterminated. The break between man and his nearest allies will then be wider, for it will intervene between man in a more civilised state, as we may hope, even than the Caucasian, and some ape as low as a baboon, instead of as now between the negro or Australian and the gorilla.
The text of The Descent of Man is undeniably racist.
Entrance gate to concentration camp Buchenwald
Truly, this earth is a trophy cup for the industrious man. And this rightly so, in the service of natural selection. He who does not possess the force to secure his Lebensraum in this world, and, if necessary, to enlarge it, does not deserve to possess the necessities of life. He must step aside and allow stronger peoples to pass him by.
The basis of Hitler’s political beliefs was a crude Darwinism.
Once the old Christian idea of a total difference in kind between man and beast has been abandoned, then no argument for experiments on animals can be found which is not also an argument for experiments on inferior men. If we cut up beasts simply because they cannot prevent us and because we are backing up our own side in the struggle for existence, it is only logical to cut up imbeciles, criminals, enemies or capitalists for the same reason.
Naturalism faces a dilemma: Either humans in some sense "transcend" nature by their moral sensibility and agency, or they are mere by-products of evolution, in which case there is no ontological ground for saying they 'ought' to behave any less aggressively than other predatory creatures in the food chain who are looking out for their own good.
In a universe of blind physical forces and genetic replication, some people are going to get hurt, other people are going to get lucky, and you won't find any rhyme or reason in it, nor any justice. The universe we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil and no good, nothing but blind, pitiless indifference.
The catastrophes of our age are the product not of necessity but of unintelligent choice.